HRSG's are popular today because of the efficiency gains that are accomplished with the re-use of waste heat from turbines.  Fundamentally HRSG's are large heat exchangers and follow the thermodynamic laws that govern heat transfer. 

The amount of heat that is available to produce steam comes from the following equation:

Q = MC _p ΔT₍₁₎

Where M is the turbine exhaust gas mass flow rate, C _pis the specific heat of the the gas, and ΔT is the difference between

the turbine outlet temperature and the stack outlet temperature. A stack outlet temperature between 280°F and 300°F is desired.  The calculation of C_p requires knowledge of the turbine exhaust gas outlet conditions.  A typical gas analysis follows:

N₂................76.0%

O₂...............14.5%

CO₂...............2.9%

H₂O...............6.6%

Minus blowdown and casing losses the Q in equation 1 is used to determine how much steam can be produced.  The following equation is used for this calculation:

M = Q/ΔH ₍₂₎

Where M is the steam mass flow rate, Q is the heat from equation 1 minus losses for b lowdown and casing, and ΔH is the change in enthalpy from feedwater enthalpy to the enthalpy of the final steam conditions. A few other concepts that are unique to HRSG's are pinch point and approach point.  See Figure 1.  All Pinch and Approach points should be sized for unfired conditions and will change during fired conditions.

Approach point is used in the sizing of the Economizer.  As you can see in fig. 1 the approach is the difference between the economizer water outlet temperature and the Saturation temperature of the the steam.  A good approach point is between 15°F-30°F.  This temperature will guarantee that no steaming will occur in the economizer section.  It is not always possible to achieve an approach point in this zone.  If the saturation temperature is too high achieving an approach temperature in this zone will lead to a stack outlet temperature well below the desired stack outlet temperature 300°F.

Pinch is used in sizing the heat transfer surface area of the HRSG.  As seen in fig. 1 the pinch point is the difference between the saturation temperature and the HRSG exit temperature.  It is desirable to make the pinch point as small as possible with out making the cost of the HRSG astronomical.  See figure 2.

As seen in figure 2 heating surface area and price rise exponentially as the pinch point approaches 0°F  A pinch point of 15°F-40°F is desired.  Both the pinch point and approach point should be chosen carefully and never set arbitrarily as this can cause temperature cross situations.  As always consult an ERI application engineer before setting final designs.